* Use rp2040-hal in all example (possibly through their bsp)
Some of the examples were using the cortex_m_rt::entry method which
misses the device specific spinlock re-initialisation.
This commits makes the usage more consistent by using rp2040_hal exported
macro as the only `entry` method used across examples.
- Clippy warns about empty loops, https://github.com/rust-lang/rust-clippy/issues/6161
- wfi allows to CPU to save some power
WFI was avoided in examples for fear of ill interactions with debuggers.
However the rp2040 debug port does continue to work, as long as the
relevant clocks are not disabled in SLEEP_EN0/SLEEP_EN1. (By default,
all clocks stay enabled in sleep mode.)
This patch replaces several different workarounds with just calling wfi.
Implementing `impl From<SystemClock> for Hertz` is a footgun, as
SystemClock is not Copy, so the automatic conversion consumes the
owned clock.
This is visible in the example i2c.rs:
```
let mut i2c = hal::I2C::i2c1(
pac.I2C1,
sda_pin,
scl_pin, // Try `not_an_scl_pin` here
400.kHz(),
&mut pac.RESETS,
clocks.peripheral_clock,
);
```
If the user wants to use both `i2c0` and `i2c1` at the same time,
copying from this example won't work:
```
error[E0382]: use of moved value: `clocks.peripheral_clock`
--> rp2040-hal/examples/i2c.rs:106:9
|
97 | clocks.peripheral_clock,
| ----------------------- value moved here
...
106 | clocks.peripheral_clock,
| ^^^^^^^^^^^^^^^^^^^^^^^ value used here after move
|
= note: move occurs because `clocks.peripheral_clock` has type
`PeripheralClock`, which does not implement the `Copy` trait
```
As getting the frequency from a clock doesn't really need ownership,
changing it to `impl From<&SystemClock> for Hertz` is both more
logical and provides better usability.
This is, however, a breaking change: Code relying on this trait
implementation needs to be changed by adding a `&`.
